It's systematic.In what way?
How would Gattuso firstly stop Sanchez fucking up a header against the Scum, then have the pace to catch Ramsey, then make Hugo narrow the angle better?
How would he make Hugo not run 40 meters off his line and conceded after 1 minute against Barca?
A DM may well cover the defense, but by your argument we could just as well upgrade Jan and Toby and also protect Hugo better.
Seeing as they are the CBs
Our system and style of play relies on a defensive midfielder that acts as a shield between the backline and the more forward running midfielders. That covers space and lessens the area that oppossing players have to work with. This in turn ensures that the defenders have more clear cut areas to work within and roles to play in the system. Which in turns means that the goalkeeper has a more defined area to work in as well. Everyone knows their role and performs it.
Our defense has seen more individual errors this season because the system has not worked. Without a clear cut DM to perform those tasks our other midfield players are stretched because they have to cover more ground, which creates more space for oppossing midfielders to work in, which in turn means that our defenders have to cover more ground and open themselves up for errors and mistakes as well.
Individual errors and brainfarts will still happen even within a well functioning system. But it is pretty clear that this year our midfield has been stretched thin which has disrupted our playing system.
We need a DM because it allows the defenders to focus on defending, and the CMs/AMs to focus on moving the ball up field.